drivers/video/backlight/lp855x_bl.c: use PAGE_SIZE for the sysfs read operation

sysfs allocates PAGE_SIZE.  It is used by each R/W operation method.  Use
it instead of another buffer size.

Signed-off-by: Milo(Woogyom) Kim <milo.kim@ti.com>
Acked-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>